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| XGID=-a-Ba-D-C---dD--ac-e---AA-:0:0:-1:13:0:0:3:0:10 | |||||
53P-43Z-51S-31 (White on roll) | |||||
In the position above, White has a 31 to play. For her play of 24/21 9/8, you chose @ (anchor). Strictly speaking, this is improper (as Igor brought to light), because @ has a dedication clause: the entire roll must be dedicated to making an anchor. with no move portion left over. (Likewise for P, the entire play must be dedicated to making a point.) For examples, see positions #6366 of the Nactation tutorial. [Without the dedication clause, @ in #66 would mean bar/23 24/22 9/5*, and the best anchoring play (making the 20pt) would be tragically demoted to third place in the @ family.]
That said, (for 24/21 9/8) you actually can get by with your @ using "assumption" (see section 8 of the same link above). White anchors with 24/21, and then plays the "obvious" ace. A computer program would not interpret the play as you intend, but it is a safe bet that any human reader would figure out that your ace is 9/8. [Your Nactation has improved. My changing @ to B in the subject header is arguably a nitpick, and the rest of your sequence is perfect.] For examples of B in action, click on the link above and see positions #17, #60 and #62.
Another proper choice for White's 24/21 9/8 play is the style letter L (Lift), though it may burden your reader with a stutter step because it is possible to lift the 9pt blot to either the 6pt or 8pt. The capital L is awarded to 24/21 9/8, which makes an extra point with the non-lifting portion, and other lifting plays (such as they are) are lower-ranked in the L family. For more on L, see positions #26 and #6970, same link as above).
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| XGID=---Bb-D-C---dD---d-e---AA-:0:0:1:62:0:0:3:0:10 | |||||
53P-43Z-51S-31B-62 | |||||
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| XGID=----b-E-C---eE---c-e----B-:0:0:1:26:0:0:3:0:10 | |||||
Reference position | |||||
[S (Split) is by itself. There is no lower-case "s" move because one of the back checkers is blocked from playing the larger number (the 6).]
........R = Run (23/15): tutorial #51 (diagrams 51.5 and 51.6).
........r = run (24/16)
........Z = reverse split (23/21 13/7): same link, #51
........z = reverse split (24/22 13/7)
........$ = Slot (13/5): same link, #2930 (and #7273)
........% = alt slot (13/11 13/7)
........S = Split (24/18 13/11): same link, #51
[You can also use W (Wild) in place of Z (and w in place of z). And it is more usual to use D in lieu of % for 13/11 13/7.]
I've studied many similar positions. For comparison, I offer the simple reference position diagrammed on the right. The only difference from the opening position is that the opponent has advanced her 24pt anchor to her 21pt.
On the right, with the back checkers not yet split, there is only one play for each of the families R, S and Z. As it happens, capital R and Z (respectively) match up to the traditional notations of the lower-case "r" and "z" in the lefthand position. (If you don't immediately grasp what I'm talking about, don't worryit's not important. I'm just making the point that Nactation is a relative system. The position on the right has not been sophisticated by the presence of a 23pt checker.)
On the right, Blue's best move is Z (reverse split), which splits with the 2 and comes down to the 7pt. Slotting in front of an advanced anchor is a great strategem. For more on this position, click here (and scroll down to the second diagram).
In the lefthand position, Blue has already gained the flexibility of split back checkers. Advancing one of these split checkers (23/21 being the better of the two options, so as not to walk another checker into the direct range of White's 8pt stack) does not gain as much as the virgin deuce split gains on the right. It is for this reason that instead playing the deuce 7/5, aiming to cover a better slotted point, becomes roughly as good as 23/21.
Another key difference in the positions is White's midpoint/8pt distribution. It is worse for Blue to get hit from White's midpoint (and less bad to get hit from White's 8pt) in the righthand position than in the lefthand position. It's the basic axiom that one should avoid putting blots in front of stacks (i.e., on points that allow the opponent to unstack as she hits). This is why running (with either back checker) is more reasonable on the left than on the right.
These factors narrow the gap between candidates and, as it happens (according to your rollout), cause four plays to be tied on the left: [Z=R=$ r1] "<=25. By contrast, on the right Z clearly beats R and $ (the evaluation being [Z S13 R33 $33 %63 U68] "&e).
A cogent variant (of the lefthand position) is to just move White's second 8pt spare back to the midpoint (diagrammed below), which isolates the effect of White's midpoint/8pt distribution. Again, Blue wants to stay out of the far side outer board, as verified by the evaluation [Z $21 r36 R37 z39 S47 %69 U80] "&e. (The reason that the non-back-checker-promoting $ does not remain tied with Z is that Blue leads by 5 extra pips in the race.)
Nack
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| XGID=---Bb-D-C---eD---c-e---AA-:0:0:1:26:0:0:2:0:10 | |||||
51S-21U-53P-21@-62 | |||||